Engineering Geological Mapping of Al Jubail City, Kingdom of Saudi Arabia

Abstract

Urban development and the expansion in the industrial areas are long faced with many geotechnical constraints, such as weak sabkha soils, mobile sand dunes, and saline groundwater. Site investigation and engineering geological mapping help in recognizing the importance of the geological conditions in the founding and growth stages of cities. The aim of this study is to conduct a detailed investigation to produce an engineering geological map and 3D soil profiles in Al Jubail Industrial complex to support urban planning, infrastructure design and hazard reduction. This was achieved by classifying the soil types, measuring the soil engineering properties and identifying the type of geological hazards within an area of 569 km 2 . Huge construction works have still going on since the 80’s which require an extra effort to match the week nature of the soil that may require foundation modification and/or soil improvement. The present investigation program starts with mapping the geological units in the area, followed by extensive in-situ testing (e.g., Standard Penetration Tests, SPT) and surface soil sampling at 23 surface points and 55 boreholes, following the American Association for Engineering Geologists (AAEG) standards. Several surface comprehensive engineering geological maps in addition to a number of soil profiles were prepared at different scales that can be used as multi-purpose. The maps took into consideration the surface modifications in the soil type and the new boundary between the various geological units. It was found that Al Jubail area is composed of sand dunes and sheets, and low laying sabkhas that pose several significant challenges for the planners and designers. The upper surface to 2 m is mainly loose poorly graded sand (SP) with low relative density, gradually changing to stronger poorly graded sand with silt (SP-SM) at greater depths, overlaid on sandy or clayey week sabkha flats. As a result, raising the ground level by landfill and rectifying the soil is performed on a large scale using either dune sand or dredged soil from the sea. The soil at the surface was divided into five engineering zones: Zone I (fine-medium sand over silty clay), Zone II (well-graded brown sand), Zone III (medium-coarse milky sand), Zone IV (sand with gravel), and Zone V (fine orange sand). SPT N-values at the upper 1.5 m ranged from 25 - 40 (moderately dense), improving noticeably below 6 m. Hazards include sand migration, sabkha corrosion and collapse, settlement in fat clay layers (CH) at 8 - 14 m, and shallow water tables (2 m near shore). 3D fence diagrams and profiles illustrate heterogeneous subsurface layers of marine origin, with relative densities from medium dense/medium stiff to very dense/very stiff. The detail in the produced engineering geological maps is proportional to their scale. The identified geological hazards include drifting sand and migrating dunes, the week and corrosive sabkhas, and the geotechnical problems of the fill materials in the form of settlement or vertical water percolation [1] . The engineering maps and the cross sections will provide general pictures of the material type and their engineering properties in Al Jubail. It is important to serve the developers, construction companies, consulting engineers, and the city residents to avoid problematic grounds or hazardous environment. These findings enhance engineering geological practices by providing a model for integrating satellite imagery, field data, and AAEG zoning in similar arid-industrial settings, enabling proactive soil improvement (e.g., compaction, deep foundations), risk avoidance for developers and residents, and sustainable infrastructure development to minimize settlement and environmental hazards.